As compared to a CRT (Cathode Ray Tube) that has been mainly used for the purpose of realizing moving images, an LCD (Liquid Crystal Display) has a drawback, that is, a motion blur, that when displaying an image in motion, outline of a moving part is perceived by a viewer out of focus. It is pointed out that the motion blur is caused by a display system itself of the LCD (see, for example, Japanese Patent No. 3295437; Hidekazu Ishiguro and Taiichiro Kurita, “Consideration on Motion Picture Quality of the Hold Type Display with an octuple-rate CRT”, Technical Report of IEICE, Institute of Electronics, Information and Communication Engineers, EID96-4 (1996-06), pp. 19-26).
In the CRT which performs displaying with the light emitted from a phosphor caused by scanning electron beams, light emission of each pixel is almost an impulse although slight afterglow of the phosphor exists. This is called an impulse-type display system. On the other hand, in the case of LCD, an electric charge accumulated by application of an electric field to a liquid crystal is retained at a relatively high rate until an electric field is applied next time. Particularly, in the case of a TFT (Thin Film Transistor) system, a TFT switch is provided for each dot that constitutes a pixel, and auxiliary capacitance is further provided for each pixel generally, thus the capability of retaining accumulated charge is extremely high. Accordingly, light is consecutively emitted until a pixel is rewritten by application of an electric field based on image information of a next frame or field (hereinafter, represented by a frame). This is called a hold-type display system.
In the hold-type display system as described above, since impulse response of image display light has a temporal spread, time frequency characteristics are worsened with the deterioration of space frequency characteristics, accompanied and therefore a motion blur occurs. That is, since human eyes smoothly follow a moving object, when the light emission time is long like in the case of the hold type, motion of an image looks jerky and unnatural due to a time integration effect.
A technology is known, that an image is interpolated between frames to convert a frame rate (the number of frames/second: fps), in order to improve the motion blur in the above-described hold-type display system. This technology is called FRC (Frame Rate Converter) and is put into practical use in a liquid crystal displaying device and the like.
Conventionally, as a method for converting a frame rate, there are various methods such as simply repeating read-out of the same frame for a plurality of times, and interpolating a frame by straight-line interpolation between frames (linear interpolation) (see, for example, Tatsuro Yamauchi, “TV Standards Conversion”, Journal of the Institute of Television Engineers of Japan, Vol. 45, No. 12, pp. 1534-1543 (1991)). However, in the case of frame interpolation processing by linear interpolation, unnaturalness (jerkiness, judder) accompanying frame rate conversion occurs, and the motion blur interference caused by the above-described hold-type display system, can not be sufficiently improved, and therefore, it is impossible to achieve sufficient image quality.
Hence, in order to improve quality of a moving image by eliminating influence of the above-described jerkiness and the like, processing of frame interpolation of a motion compensation type (motion compensation) using a motion vector is proposed. According to the motion compensation processing, a moving image itself is captured to perform compensation, thus making it possible to obtain a highly natural moving image without deteriorating resolution or generating the jerkiness. Further, since an interpolation image signal is formed with motion compensated, it is possible to sufficiently improve the motion blur interference caused by the above-described hold-type display system.
In the above-described Japanese Patent No. 3295437, a technology is disclosed, that by generating an interpolation frame motion-adaptively, a frame frequency of a display image is increased to improve deterioration in space frequency characteristics that causes a motion blur. In this technology, at least one interpolation image signal that is interpolated between frames of the display image is formed motion-adaptively from previous and subsequent frames so that the formed interpolation image signal is interpolated between frames and displayed sequentially.
FIG. 1 is a block diagram showing the schematic structure of an FRC driving display circuit in a conventional liquid crystal displaying device, and as shown in the figure, the FRC driving display circuit is constituted by including an FRC portion 100 for converting the number of frames of the input image signal by interpolating an image signal (a frame displayed in gray) which motion compensation processing is executed between frames of an input image signal, an active-matrix liquid crystal display panel 104 that has a liquid crystal layer and an electrode for applying a scanning signal and a data signal to the liquid crystal layer, and an electrode driving portion 103 for driving a scanning electrode and a data electrode of the liquid crystal display panel 104 based on an image signal to which frame rate conversion is performed by the FRC portion 100.
The FRC portion 100 includes a motion vector detecting portion 101 for detecting motion vector information from an input image signal, and an interpolation frame generating portion 102 for generating an interpolation frame based on the motion vector information obtained by the motion vector detecting portion 101.
In the above-described structure, the motion vector detecting portion 101 may obtain motion vector information, for example, using a block matching method, a gradient method, or the like, which will be described below, or when motion vector information is included in an input image signal in some way, this may be used. For example, since image data that is compressively encoded using an MPEG (Moving Picture Experts Group) system includes motion vector information of a moving image calculated in encoding, the structure to acquire the motion vector information may be employed.
FIG. 2 is a view illustrating frame rate conversion processing by the conventional FRC driving display circuit shown in FIG. 1. The FRC portion 100 of FIG. 1 generates interpolation frames between frames by motion compensation using motion vector information output by the motion vector detecting portion 101, and sequentially outputs the generated interpolation frame signal with an input frame signal. By means of this, processing of converting a frame rate of an input image signal, for example, from 60 frames per second (60 Hz) into 120 frames per second (120 Hz) is performed.
FIG. 3 is a view illustrating interpolation frame generation processing by the motion vector detecting portion 101 and the interpolation frame generating portion of FIG. 1. The motion vector detecting portion 101 detects a motion vector 105, for example, from a frame #1 and a frame #2 shown in FIG. 3 with a gradient method or the like. That is, the motion vector detecting portion measures a direction and an amount of motion in 1/60 second between the frame #1 and the frame #2 to obtain the motion vector 105. Next, the interpolation frame generating portion 102 uses the obtained motion vector 105 to assign an interpolation vector 106 between the frame #1 and the frame #2. By moving an object (an automobile in FIG. 3) from a position of the frame #1 to a position after the elapse of 1/120 second based on the interpolation vector 106, an interpolation frame 107 is generated.
In this way, by performing motion compensation frame interpolation processing using motion vector information to increase a display frame frequency, a display state of LCD (hold-type display system) can be put close to a display state of CRT (impulse-type display system), and it becomes possible to improve deterioration in image quality due to a motion blur caused when displaying a moving image.
Here, in the above-described motion compensation frame interpolation processing, it is essential to detect a motion vector for motion compensation. As the representative method for detecting a motion vector, for example, a block matching method, a gradient method, and the like are proposed. In these methods, a motion vector is detected for each pixel or small block between two consecutive frames, and each pixel or each small block of an interpolation frame between two frames is interpolated using the motion vector. That is, an image at an arbitrary position between two frames is interpolated at an corrected position to convert the number of frames.
Meanwhile, since a moving image has high correlation between frames and has continuity in a time axis direction, a pixel or a block moving in a certain frame is moved with a similar motion amount also in subsequent frames or previous frames in many cases. For example, in the case of a moving image in which a state that a ball rolls from right to left in a screen is photographed, an area of the ball is moved while having a similar motion amount in any frame. That is, between consecutive frames, a motion vector has continuity in many cases.
Accordingly, by referring to a detection result of a motion vector in a previous frame, it is possible to detect a motion vector in a subsequent frame more easily or more accurately. For example, an iterative gradient method, which is an improved version of a gradient method, uses a method in which calculation of the gradient method is repeated to a block to be detected by defining a motion vector of a neighboring block that has been detected in a previous frame or a current frame as an initial displacement vector and deciding it as a starting point. According to this method, it is possible to obtain a motion amount almost accurately by repeating the gradient method about twice.
Moreover, in a block matching method, it is considered to detect a motion vector efficiently by changing a search order referring to a detection result of a motion vector in a pervious frame. In this way, when detecting a motion vector, by using an already-detected motion vector, for example, it is possible to perform frame rate conversion in real time.
On the other hand, as the source of a video signal to be displayed on an image displaying device, in addition to a video by a general television broadcast, there also exists a video reproduced/transmitted by an externally connected video reproducing device (for example, a DVD (digital versatile disc) player, an HD (hard disc) player, or the like) or a video reproducing device built in the displaying device. Generally, a video reproducing device has special reproduction functions such as “fast forward reproduction” (high-speed and forward-direction reproduction), “rewind reproduction” (high-speed and reverse-direction reproduction), “slow reproduction”, “frame advance reproduction”, and “frame back reproduction”, which can be instructed and operated by a user, and there is a case where an image signal converted into a special reproduction video by the video reproducing device is input.
Moreover, in recent years, with the progress of recording capacity of a recording medium (for example, a DVD, an HD and the like) and further more digitization of a transmission system, as a recording form of a video signal, for example, a digital compression signal formed by an MPEG system, or the like in which correlation between frames is removed is used.
The structure of the MPEG system represented by a digital compression signal will be described below. The prediction structure of inter frame compression used in the MPEG system is composed of I frame compressed only with data in a frame, P frame compressed by taking a difference from I frame or P frame, ahead, and B frame compressed by taking a difference from I frame or P frame, ahead and behind.
I frame is capable of restoring an image with data of itself, however, P frame and B frame need to restore an image using a frame defined as a reference (use for taking a difference) in compression. When a video signal recorded in the MPEG system is subjected to special reproduction such as “fast forward reproduction” and “rewind reproduction” by a video reproducing device, a method for reproduction by picking up only I frame capable of restoring only with data in the frame, a method for reproduction by generating an image for special reproduction using I, P, and B frames, and the like are realized (for example, refer to Japanese Laid-Open Patent Publication No. 8-130708).